THE GLACIERS OF MOUNTAIN AND CONTINENT

Conditions essential to glaciation.—Wherever for a sufficiently protracted period the annual snowfall of a district is in excess of the snow that is melted, a residue must remain from each season to be added to that of succeeding ones. Eventually so much snow will have accumulated that under its own weight and in obedience to its peculiar properties, a movement will begin within the mass tending to spread it and so to reduce the slope of its upper surface (Frontispiece plate). The conditions favorable to glaciation are, therefore, heavy precipitation and low annual temperature. If the precipitation is scanty, the small snowfall is soon melted; and if the temperature be too high, the moisture is precipitated not in the form of snow but as rain. It is important here to keep in mind that snow is a poor heat conductor and itself protects its deeper layers from melting.

The snow-line.—Because of the low temperatures glaciers should be most abundant or most extensive in high latitudes and in high altitudes. The largest are found in polar and subpolar regions, and they are elsewhere encountered only at considerable elevations. The largest glaciers are the vast sheets of ice which inwrap the continents of Greenland and Antarctica, but glaciers of large size are to be found upon other large land masses of the Arctic, as well as in Alaska, in the southern Andes, and in New Zealand. Much smaller glaciers are characteristic of certain highlands within temperate and tropical regions, but because of specially favorable conditions both of altitude and precipitation the Himalayas, although in relatively low latitudes, nourish glaciers of large proportions. In general, it may be said that the nourishing grounds of glaciers are largely restricted to those areas where snow covers the ground throughout the year. The lower margin of such areas is designated the snow line, and varies but little from the line on which the average summer temperature is at the freezing point of water—the so-called summer isotherm of 32° Fahrenheit. Within the tropics this line may rise as high as 18,000 feet above the sea, whereas in polar latitudes it descends to sea level.

Importance of mountain barriers in initiating glaciers.—The precipitation within any district depends, however, not alone upon the amount of moisture which is brought to it in the clouds, but upon the amount which is abstracted before the clouds have passed over it. The capacity of space to hold moisture increases with its temperature, and hence any lowering of this temperature will reduce the capacity. If lowered sufficiently, the point of complete saturation will be reached and further cooling must result in precipitation. Hence, anything which forces an air current to rise into more rarefied zones above, will lower the pressure upon it and so bring about a cooling effect in which no heat is abstracted. This so-called adiabatic refrigeration of a gas may be illustrated by the cool current which issues in a jet from a warm expanded rubber tire after the cock has been opened; or even better, by the instant solidification at extreme low temperatures of such normal gases as carbonic acid when they are allowed to issue under heavy pressure from a small orifice.

As applied to moisture-laden and near-surface winds, the effective agents of adiabatic cooling are the upland areas upon the continents, and especially the ranges of mountains. These barriers force the moving clouds to rise, cool, and deposit their moisture. It is, therefore, the highland barriers which face the oncoming, moisture-laden winds that receive the heaviest precipitation. Within temperate regions, because of the prevalence of westerly winds, those barriers which face the western shores receive the heaviest fall. Within the tropics, on the other hand, it is the barriers facing the eastern shores which, because of the easterly “trades”, are most favorable to precipitation.

Thus it is in the Sierra Nevadas of California, and not in the Rockies or the Appalachians, that the glaciers of the United States are found. The highland of the Swiss Alps lying likewise athwart the “westerlies” of the temperate zone acquires the moisture for nourishment of its glaciers from the western ocean—here the Atlantic (Fig. 291). Within the tropics the conditions are reversed, and it is in general the ranges which lie nearer the eastern coasts that are the more favored. If no barrier is found upon this coast, the clouds may travel over vast stretches of country before being arrested by mountains and robbed of their moisture. Thus in tropical Brazil the glaciers are found in the Andes upon the Pacific coast though nourished by clouds from the Atlantic.

Sensitiveness of glaciers to temperature changes.—How sensitive is the adjustment between snow precipitation and temperature may be strikingly illustrated by the statement on excellent authority that if the average annual temperature of the air within the Scottish Highlands should be lowered by only three degrees Fahrenheit, small glaciers would be the result; and a moderate temperature fall within the region surrounding the Laurentian lakes of North America would bring on glaciation, otherwise expressed as a depression of the snow line of the region.

The cycle of glaciation.—Though to-day buried beneath its ice mantle, it is known that Greenland had more than once in earlier geological ages a notably mild climate, and in some future age it may revert to this condition. In other regions, also, we have evidence that such a rotation of climatic changes has been successively accomplished, the climate having steadily increased in severity towards a culminating point, and been followed by a reverse series of changes. Such a complete period may be called a cycle of glaciation. While the climate is steadily becoming more rigorous, we have to do with an advancing hemicycle of glaciation, but after the culminating point has been reached, the period of amelioration of climate is the receding hemicycle.

The advancing hemicycle.—There is little reason to doubt that whatever be the cause of the climatic changes which bring on glacial conditions, these changes come on by insensible gradations. The first visible evidence of the increased severity of the climate is the longer persistence of the winter snows, at first within the more elevated districts. In such positions drifts must eventually continue throughout the warm season and so contribute to the snow accumulations of the succeeding winter. This point once reached, small glaciers are inevitable, even should the average temperature fall no further, for the snow left over in each season must steadily increase the depth of the deposits until the weight brings about an internal motion of the mass from higher to lower levels.

The inherited depressions of the upland—the gentle hollows at the heads of rivers—will first be filled, and so the valleys below become the natural channels for the outflow of the early glaciers. With a continued lowering of the annual temperature and consequent increased snowfall, the early glaciers become more and more amply nourished. Snow and ice will, therefore, cover larger areas of the upland, and the glaciers will push their fronts farther down the valleys before they are wasted in the warm air of the lower levels. As the valleys become thus more completely invested by the glacier they are likewise filled to greater and greater depths, and they may thus submerge portions of the walls that separate adjacent valleys. Reaching at last the front of the upland area, the glaciers may now be so well nourished at their heads that they push out upon the flatter foreland and without restraint from retaining walls spread broadly upon it (Fig. 292).

The culmination of the progressive climatic change may ere this have been reached and milder conditions have ensued. If, however, the severity of the climate should be still further increased, the expanded fronts of neighboring glaciers will coalesce to form a common ice fan or apron along the foot of the upland (Plate 18 B). This could hardly take place without a still further deepening of the ice within the valleys above, and, probably, a progressive submergence of the lower crests in the valley walls. This may even continue until all parts of the upland area have been buried. The snow and ice now take the form of a covering cap or carapace, and the upper portions being no longer restrained at the sides, now spread into a broad dome, as would a viscous liquid like thick molasses when poured out upon the floor (Fig. 293). The lower zones of the mass and the thinner marginal portions still have their motion to a greater or less extent controlled by the irregularity of the rock floor against which they rest.

The reverse series of changes in the glacier is inaugurated by an amelioration of the climate, and here, therefore, the advancing hemicycle becomes merged in the receding hemicycle of glaciation.

Continental and mountain glaciers contrasted.—The time when the rock surface becomes submerged beneath the glacier is, as regards both the surface forms and the erosive work, a critical point of much significance; for the ice cap and larger continental glacier obviously protect the rock surface from the action of those chemical and mechanical processes in which the atmosphere enters as chief agent, and which are collectively known as weathering processes. Until submergence is accomplished, larger or smaller portions of the rock surface project either through or between the ice masses and are, therefore, exposed to direct attack by the weather (see below, p. 370).

Snow which falls in the mountains is not allowed to remain long where it falls. By the first high wind it is swept off the more elevated and exposed surfaces and collected under eddies in any existing hollows, but especially those upon the lee slopes of the range. We are to learn that glaciers carve the mountains by enlarging the hollows which they find and producing great basins for the collection of their snows; but with the initiation of glaciation the inherited hollows are in most cases the unimportant depressions at the heads of streams. Whatever they may be and however formed, the snow first fills those hollows which are sheltered from the wind, and as it accumulates and becomes distributed as ice, assumes a surface of its own that is dependent upon the form and the position of the basin which it occupies (see Fig. 294).

When the quantity of accumulated snow is so great that all hollows of the rock surface are filled, its own surface is no longer controlled by retaining rock walls, and it now assumes a form largely independent of the irregularities in the upland. Experience shows that this surface is approximately that of a flat dome or shield, and as it covers all the upland, save where the ice thins upon its margins, this type of glacier is called an ice cap (Fig. 295). All types of glacier in which rock projects above the highest levels of the ice and snow are known as mountain glaciers.

The flat domes of ice which mantle the continents of Greenland and Antarctica, though resembling in form the smaller ice cap, are yet because of their vast size so distinct from them, particularly in the manner of their nourishment, that they belong in a separate class described as inland ice or continental glaciers. Though they have some affinities with ice caps, they are most sharply differentiated from all types of mountain glaciers. Of them it is true that the lithosphere projects through them only in the neighborhood of their margins (Fig. 296), whereas in the case of mountain glaciers rock may project at any level but always above the highest snow surface. Ice caps may be regarded as intermediate between the two main classes of mountain and continental glaciers (Fig. 297). Because of the large rÔle which continental glaciers have played in geological history, it is thought best to consider them first, leaving for later discussion the no less interesting but less important mountain glaciers.

The nourishment of glaciers.—The life of a glacier is dependent upon the continued deposition of snow in aggregate amount in excess of that which is lost by melting or by other depleting processes. Whenever, on the other hand, the waste exceeds the precipitation, the glacier is in a receding condition and must eventually disappear, if such conditions are sufficiently long continued. The source of the snow is the water of the ocean evaporated into the atmosphere and transported over the land in the form of clouds. We are to learn that the changes which this moisture undergoes before its delivery to the glacier are notably different for the classes of continental and mountain glacier.

The upper and lower cloud zones of the atmosphere.—Before we can comprehend the nature of the processes by which glaciers are nourished, it will be necessary to review the results of recent studies made upon the earth’s atmospheric envelope. It must be kept in mind that the sun’s rays are chiefly effective in warming the atmosphere through being first absorbed by some solid body such as rock or water and their heat then communicated by contact to the immediately adjacent air layers. The layers thus warmed being now lighter than before, they rise and are replaced by colder air, which in its turn is warmed and likewise set in upward motion. Such currents developed in the air by contact with warmer solid bodies constitute the process known as convection.

To a relatively small degree the atmosphere is heated by the direct absorption of the sun’s rays which pass through it. Since air has weight, it compresses the lower layers near the earth, and hence as we ascend from the earth’s surface the air becomes continually lighter. Convection currents must, therefore, adjust themselves by the air expanding as it rises. But expansion of a gas always results in its cooling, as every one must have observed who has placed his finger in the air current which escapes from the open valve of a warm rubber tire. Dry air is cooled a degree Fahrenheit for every six hundred feet of ascent in the atmosphere. At a height of about seven miles above the earth’s surface all rising air currents have cooled to about 68° below the zero of the Fahrenheit scale, and exploration with balloons has shown that the currents rise no farther. At this level they move horizontally, just as rising vapor spreads out in a room beneath the ceiling. Above this level, as far as exploration has gone, or to a height of more than twelve miles, the temperature remains nearly constant, and this upper zone is, therefore, called the isothermal or the advective zone—the uniform temperature zone of the lower atmosphere. Beneath the convective ceiling the process of convection is characteristic, and this zone is therefore described as the convective zone (Fig. 298).

A large part of the moisture which rises from the ocean’s surface is condensed to vapor before it has ascended three miles, and in this form it makes its transit over land as fleecy or stratiform clouds—the so-called cumulus and stratus clouds and their many intermediate varieties (see Frontispiece). This lower layer within the convective zone is, therefore, a moist one overlaid by a relatively drier middle layer of the convective zone. That moisture which rises above the lower cloud layer is congealed by adiabatic cooling to fine ice needles visible as the so-called cirrus clouds which float as feathery fronds beneath the convective ceiling (see frontispiece at right upper corner of picture). Thus we have within the convective zone an upper layer more or less charged with water in the form of ice needles. It is the clouds of the lower zone whose moisture in the form of vapor supplies the nourishment of mountain glaciers, and the high cirrus clouds whose congealed moisture, after interesting transformations, is responsible for the continued existence of continental glaciers.

As we are to see, there are other noteworthy differences between continental and mountain glaciers, in the manner of their sculpture of the lithosphere, so that long after they have disappeared the characters of each are easily identified in their handiwork. How the lower clouds are forced upward and so compelled to give up their moisture to feed the mountain glaciers, and how the upper clouds are pulled downward to nourish the glaciers of continents, can be best understood after the characteristics of each glacier class have been studied.